place 2 quart shallow container in freezer. In medium pan, mix sugar and one 2/3 cup water. Bring to boiling and boil two minutes, stirring occasionally, until sugar is dissolved. Remove from heat. Scrape seeds from vanilla bean into syrup. Add vanilla bean pod. Cool to room temperature. Remove pod.
Whisk in mascarpone, lemon zest, and lemon juice. Refrigerate mixture two hours or until cold. Pour mixture into ice cream maker. Process according to manufacturer's instructions. Transfer sherbet to 2 quart container in freezer. Cover surface of sherbet with plastic wrap. Place lid on container. Free sherbet up to two weeks. To make without ice cream maker, pour chilled sherbet mixture into 2 quart container in freezer. Freeze 1 1/2 hours. Push semi frozen edges of sherbet into center and stir to incorporate. Repeat procedure over next three hours, or until sherbet is soft